Internal combustion engines used for both mobile and stationary applications are subject to strict emission limits. One approach to reducing emissions is to improve in-cylinder designs, but these improvements have fallen short of meeting emissions limits. Other approaches involve exhaust aftertreatment devices, which have achieved significant emissions reductions.
For diesel engines, the main pollutants of concern are nitrogen oxides (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by the sulfur in fuel, and organic components of unburned fuel and lubricating oil.
To reduce NOx, one approach is the use of NOx reduction catalysts, such as lean NOx traps (LNTs). To reduce PM, one approach is the use of various types of diesel particulate filters (DPFs). These devices may be used alone or together, with either or both being used downstream of the engine, in the exhaust line. Both operate in repeated loading and regeneration cycles.
For regeneration of NOx reduction catalysts, the oxygen content in the exhaust is reduced. This reduced oxygen content is usually combined with increased exhaust hydrocarbon content, to obtain the rich mixture needed for the NOx regeneration process.
A diesel oxidation catalyst (DOC) is often placed upstream from a LNT. The purpose of the DOC is to condition the exhaust hydrocarbon or reform it to obtain the ideal reductant for LNT regeneration. Indications are that DOC performance improves with increased engine speed, airflow, and hence oxygen content.
In addition, for regeneration of DPFs, elevated exhaust gas temperature and increased oxygen content both contribute to increased regeneration activity. For this purpose, a DOC is often placed upstream of a DPF to form NO2 from NO.